The overall objective of the proposed research is to determine mechanism(s) responsible for dietary-induced thermogenesis. In a preliminary experiment, adult dogs were fed 80% more energy than required to maintain their body weight. These dogs initially gained body weight, but after 30 to 40 days of overeating their body weight plateaued even though they continued to overeat by 80%. Thus, these dogs markedly increased their energy expenditure as a result of chronic overfeeding. We plan to exploit this model to determine mechanism(s) responsible for dietary-induced thermogenesis. Our working hypothesis is that increased heat production in chronically-overfed dogs is related to an increase in sodium pump activity and/or protein turnover. We will evaluate the number of Na ion, K ion-ATPase enzyme units and the rate of whole body protein synthesis in control and overfed dogs. Since thyroid hormones and catecholamines are intimately associated with the regulation of energy expenditure, we will also evaluate several characteristics of these hormones in overfed dogs. Energy expenditure of the dogs will be evaluated by indirect calorimetry. Body composition will be determined. A biochemical explanation of why overeating leads to increased heat production in many individuals may help explain why only moderate overeating can lead to obesity in some individuals.